The invention pertains to the field of processing digitally encoded composite color television signals and more particularly to digitally encoding information related to color burst phase and the beginning of data on each horizontal line.
In systems for processing digitally encoded color television signals it is necessary to know when the first word of digital data defining information pertaining to each horizontal line of video signal will arrive in a stream of data. Each horizontal scan line of video data has associated therewith a group of three synchronizing signals: horizontal blanking, horizontal line synchronization and color burst. The color burst synchronizing signal comprises about eight cycles of the subcarrier signal which is used by the video processing equipment to maintain the local subcarrier oscillator in correct phase relationship with the subcarrier oscillator of the equipment that generates the original video signals. As is well known, the subcarrier signal is utilized to encode the color information into signals which are used by the video processing equipment to reproduce the color content of the original signal.
It is important to know the precise phase relationship of the color burst to the horizontal synchronizing signal at the beginning of each horizontal scan line so that the color signal can be correctly decoded. If this process is not done correctly, then the color of the resulting video signal will not match the color in the original signal.
The synchronizing signal pertaining to each horizontal scan line of video data is a composite of the three synchronizing signals mentioned above. The horizontal line synchronizing signal, further referred to as Hsync, is superimposed upon the horizontal blanking signal. The color burst signal is superimposed upon the horizontal blanking signal following the trailing edge of Hsync.
The phase of the first cycle of burst signal relative to the leading edge of the Hsync differs from line to line. In the NTSC standard television signal system the burst phase relative to the leading edge of Hsync changes by 180 degrees on every consecutive line, such that on every alternative line the burst phase is the same. In the PAL standard signal system the burst phase changes substantially by 90 degrees on consecutive horizontal lines.
In a digital video signal processing system the burst phase must be known for every horizontal scan line since typically such systems do not digitize the original synchronizing signals with the digitized video data. A thusly obtained "abbreviated" form of digital video signal satisfies the synchronization requirements for transmitting or recording digital video signals. Normally new synchronizing signals are generated by the digital video processing devices and reinserted into the digital signal before it is converted back into analog form, as it is necessary for example for display.
In the known devices three or four signals are generated and routed to every location throughout the system where synchronizing signals must be reinserted into the digital video signal. On each such location a decoding circuit is utilized to generate the necessary synchronizing signals from these three or four signals. For example in both the NTSC and PAL systems the following three signals are utilized. The first signal is a binary signal having a frequency of Hsync. It includes a pulse whose leading edge indicates that at some known number of clock cycles later a first data word of the information signal will arrive. The second signal is a clock utilized for sampling the video signal and the third signal is a binary signal which is derived from the first signal. That third signal has one binary state during an entire horizontal line interval where the burst phase is zero degrees and the other binary state for the entire line interval where the burst phase is 180 degrees. Thus the frequency of the third signal is equal to one-half of the frequency of the first signal. In
systems an additional fourth signal is utilized that has one binary state for the first two lines in every set of four consecutive horizontal lines and has the opposite binary state in the last two lines in every set of four consecutive lines. The burst phase and the beginning of video data on each horizontal line are decoded from these signals at every location where it is necessary to reinsert synchronizing signals into the digital video.
The disadvantage of this prior art approach is that it increases the complexity of the system by routing these three or four signals to many locations and utilizing a decoding circuit at each such location.
Further if delays occur in the propagation of these signals their timing relationships may become skewed such that an even more complex decoding is required to compensate for the various delays.
The present invention eliminates the above-indicated disadvantages by providing a circuit for encoding the data position and burst phase for each horizontal line at a single location in the system and by transmitting that encoded signal to all locations where such information is needed.